3G (3rd generation) communication systems will enable new kinds of calls with new features by providing packet switched (PS) connections via PS networks. Such calls can be in particular calls in the course of which multimedia content is transmitted, e.g. video calls, gaming sessions, etc.
According to 3GPP (3rd generation partnership project), which offers specifications for 3G systems, a Session Initiation Protocol (SIP) is to be employed for call control and signaling functions when Internet Protocol (IP) technology is to be used end-to-end for delivering multimedia content to mobile terminals.
SIP is an IETF (Internet Engineering Task Force) protocol which is defined e.g. in the Request for Comments (rfc) 2543bis-09: “SIP: Session Initiation Protocol”, of Sep. 4, 2000, which is incorporated by reference herein.
SIP constitutes an application-layer control protocol that can establish, modify and terminate multimedia sessions or calls between two or more parties in an IP network. To this end, a variety of SIP messages are defined. A SIP message can be either a request from a client to a server, or a response from a server to a client. SIP allows for instance to determine the willingness of a called party to engage in a requested communication and to set up a call by establishing call parameters at both, called and calling party.
However, also conventional circuit switched systems will further exist within the 3G communication systems. The 3G specific features relying on a PS connection can obviously not be used with communication devices connected to a circuit switched (CS) network of the communication system.
A communication device may be connected to a circuit switched network of a 3G communication system, because it is a CS device which is only capable of accessing circuit switched networks. But also PS devices may roam from a PS network to a CS network, e.g. from UMTS (universal mobile telecommunication services) to GSM (global system for mobile communications), in particular as long as the PS networks do not cover all areas.
A calling party may therefore unintentionally try to establish a 3G specific session with a called party which is at least currently connected to a CS network. The calling party is not able to know to which kind of network a called PS device is currently connected, and sometimes, a calling party may not be aware that the called party is using a CS device.
When a session requiring a PS connection is requested to be establish with such a device connected to a CS network, an establishment of the connection will be tried, but fail and be dropped. This implies a waste of resources of the connecting networks and of the device connected to a CS network. Further, the user of the requesting terminal is not aware of the reason for which the requested session is not established.